Powersupply Issue?

Hey guys I'm really hoping you can help me out here im frankly puzzled by what's happening with my PC.

A few weeks ago we had a huge power surge due to lightning and it blew my old PSU. I replaced my old PSU with a second hand one from a computer manufacturer due to replacing this pc in about 4-6 months time and not wanting to spend much. Everything worked fine for about 2 weeks but since then everytime I turn on the pc it begins the boot process and lasts maybe 5 seconds (nothing comes up on the monitor). It will then make an odd scraping noise and shuts down completely, then it will automatically turn itself back on and start booting again, this process will be repeated constantly and randomly until it finally post's normally after perhaps 2-3 minutes of this. Once it has posted there's no further issues the pc runs fine even with stressful things like games and all temps are normal.

The reason im posting here is I suspect it is the PSU causing this, I don't know the brand unfortunatly I was a bit desperate just to get the pc back up and running for work reasons and just took his most powerful which was a 650w from an existing computer he had in the back. The only other thing I wondered if it could be is my CPU fan it doesn't sit very securely on the heatsink its old and something snapped a while ago on it.

Any help on this would be greatly appreciated, i'm concerned it could be something worse.

Specs

Intel Core 2 Duo - 2.6ghz
Corsair Dominator DDR2 - 2048
Geforce 8800 Gt - 512
Board: Gigabyte Technology Co., Ltd. EP45-UD3LR